Strange hot-cold crossover situation

[Mooch91]

All,

I have an 8-year-old home with a 4-year-old Navien tankless water heater. Plumbing is CPVC throughout the house. Water supply is a well, with a 45 psi pressure regulator downstream of the well expansion tank (to help temperature swings with the water heater). Water heater is set to 130 deg F, service is propane. There is no expansion tank on the cold side of the water heater.

I have a strange crossover situation that I observe periodically. After an extended period of time, I notice that turning on a cold supply tap at one of the far reaches of the home produces hot water for a period of about 15-30 seconds. I would estimate the water temperature to be 100-105 deg F or so.

For example, I showered last night at 10:30PM and went to bed. Ran no water throughout the night. In the morning at 6:30AM, I ran a cold water faucet in another bathroom and the water was hot.

This is strange to me for a couple of reasons.

Being a tankless system, and being that the cold faucet produces this hot water almost immediately, it leads me to believe that the hot water is already in the pipes when I open the faucet. The time it takes for me to feel the hot water when I open up the cold tap is much, much faster than it normally even takes the water heater burner to light. There is usually a lag of up to 30 seconds to get water when I open a hot faucet.

That said, I find it hard to believe that the pipes are keeping water this hot for 8 hours, which is the usual amount of time between last use of hot water and when I notice this issue.

Another observation, which may or may not be related, is that I'm hearing some fairly loud knocking in the pipes when running water for the first time in the morning. It also occasionally occurs during the day as well. I can't tell if it's supply or drain, but it starts a few seconds after the water starts running and stops immediately when I shut it off. I never notice any air or spitting from the faucets when this occurs.

Any thoughts are appreciated, thanks.

 

[Reach4]

Do you have a hot water recirc system? Some of them put hot water into the cold pipe on purpose. They do this to not need a separate return line.

 

[Sponsor]

 

[Mooch91]

Do you have a hot water recirc system? Some of them put hot water into the cold pipe on purpose. They do this to not need a separate return line.

No this is not a recirc system. I'll get the model number of the heater when I get home.

 

[Dana]

Is the cold supply plumbing to that tap in the attic, above the insulation? Daytime attic temps in the 120s F or higher are pretty common this time of year, and even at night recently the outdoor lows haven't been low enough to cool the attic quickly.

 

[Mooch91]

Is the cold supply plumbing to that tap in the attic, above the insulation? Daytime attic temps in the 120s F or higher are pretty common this time of year, and even at night recently the outdoor lows haven't been low enough to cool the attic quickly.

No, should not be (I would probably have a freezing issue in the winter here in PA!). It's all on interior walls, coming up through the basement. I believe I've experienced it during the cooler months as well.

 

[Mooch91]

The only thing I can think of is that the hot water is some how bleeding back in to the cold supply when it shuts down and flowing to high points in the plumbing. I wasn't sure if the pressure regulator in-line could be contributing in that it prevents the backflow in to the well pressure tank, and instead causes the pressure to build in the cold plumbing (and the hot water to bleed in).

I would think if it were a faucet causing the crossover, that it would happen more in operation than it would when everything is shut off.